I agree with subbie. (as does the natural world, I might add
)
If evolution is defined as "descent with modification" then it's occurence is as much of a fact as just about anything, hardcore epistemological skepticism aside.
Again, as subbie said, we've observed it in nature and in the lab.
While there is plenty of evidence for it, I do suppose I can reasonably understand why some people might not think the idea that all current life evolved from a common ancestor should be regarded as a fact though. We obviously couldn't have
directly observed that.